Inconsequent shading problem. Tipps for a Beginner
Enter edit mode. Press "3" to filter to FACES. Press "A" to select all faces, click SHIFT+N (recalculate normals) and this will make all your faces coplanar to the average facing normals.
Second thing to check: Are all of your polygonal faces connected? Because they look like they don´t. Join each vertex by selecting 2 or more vertex and press ALT+M, select "center" and this will join the vertices for your poly face.